The Magnetism of Frustrated RFe₆Al₆ Compounds Studied by μSR and Mössbauer Spectroscopy

Abstract

μSR studies on the RFe₆Al₆ (R = Tb, Ho, Er) ferrimagnets (T _N≈340 K) show that the presence of frustration by competing exchange causes a slow approach of the ordered spins towards their static limit. This explains the non-Brillouin-like behavior of the magnetic Bragg peak intensities seen in neutron diffraction. Furthermore, considerable local spin disorder is found, despite the existence of long-range order. In YFe₆Al₆, having no moments on the normally strongly-magnetic R sublattice, neutron diffraction was unable to detect any magnetic Bragg peaks. The μSR and Mössbauer spectra however, clearly reveal a magnetic transition near 340 K, but not all of the Fe ions order at this point. The Fe ions on the mixed Fe-Al sublattice (8j) order first, while the pure Fe sublattice (8f) orders only below ∼70 K. The Fe ions on this site must be inherently frustrated which is also reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations.